1. Field of the Invention
The present invention is related to a circuit for detecting a knee of a waveform, and more particularly related to a circuit for detecting a knee of a waveform by using two comparators to detect the knee of the voltage waveform and the total oscillating time.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional isolated load circuit. As shown, a driving circuit PA1 is utilized for driving the isolated load circuit PA2. The isolated load circuit PA2 includes an isolated transformer PA21, which includes a primary side winding PA211, a secondary side winding PA212, and an auxiliary winding PA213, a switch PA22, and at least one operating element PA23, which is a light emitted diode in general and three operating elements are showed in the figure. The switch PA22 is electrically connected to the primary side winding PA211 of the isolated transformer PA21. The operating elements PA23 are electrically connected to the secondary side winding PA212 of the isolated transformer PA21. The electrical connection of the isolated load circuit PA2 is well known in the art and thus is not repeated here.
Please also refer to FIG. 2, which is a waveform diagram showing voltage of the conventional isolated transformer PA21. As shown, when the switch PA22 is turned off, the energy store in the primary side winding PA211 would be transferred to the secondary side winding PA212 and the auxiliary winding PA213 of the isolated transformer PA21 making use of Faraday's law. A stable average current Ia is necessary for the operating elements PA23. Because the auxiliary winding PA213 and the secondary side winding PA212 have the same winding direction, the voltage difference between two sides of those windings would be proportional to the number of turns when the two windings as the switch PA22 is turned off. The voltage of the secondary side winding PA212 is further divided by the resistors PA24 and PA25 to generate the voltage at the end of the zero crossing detecting (ZCD) of the driving circuit PA1. The knee detected at the end of ZCD, the driving circuit PA1 can be used to determine the amount of energy in the secondary side winding PA212 of the isolated transformer PA21 for deciding on/off time of the switch PA22 to achieve the average current. In the other cases, the voltage level at the FB end, i.e. the feedback voltage, can be used to determine the on/off time of the switch PA22. However, as the switch PA22 is turned off, the knee detected at the end of ZCD, the driving circuit PA1 would be influenced by the primary side winding PA211 and the noise on the switch PA22, such that the voltage oscillating waveform 100 would be generated and further result in the unstable average current Ia. Accordingly, an improvement for stabilizing the average current Ia is highly desirable.